5 Answers
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According to http://www.writersblock.ca/fall2000/origins.htm, it refers to the practice of heraldry. It comes from the sense of 'horn' as a trumpet, and one who blows his own horn is someone making great fanfare about himself, as is usually more appropriately left to a herald.

Matthew 6:2-4
2“Thus, when you give to the needy, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by others. Truly, I say to you, they have received their reward. 3But when you give to the needy, do not let your left hand know what your right hand is doing, 4so that your giving may be in secret. And your Father who sees in secret will reward you.

Are you implying that Matthew 6 is the actual historical origin of "to blow your own horn"? Is there any evidence to support such a theory? For example can you show it not used in English before the Bible was translated into English? Please flesh out your answer.
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MετάEdMar 4 '14 at 18:48

Anyone who has trouble understanding where this idiom comes from obviously never shared quarters with a sixth-grader who was learning to play trumpet. ;^)

In addition to blow/toot your own horn/trumpet, there's also the idiom beat your own drum.

Beat your own drum. Toot your own horn.1

Everyone's right could be someone's wrongBeat your own drum scream your own song2

Either one means "draw some attention to yourself." Usually, the easiest way to do that is to make a lot of noise, to clang your own cymbals. Although, if you really want to catch someone's attention, whisper.3

1 Bob Schumacher, SOLUTIONS2Teresa Taylor, Love Poetry3Tag line for an advertising campaign for Coty perfume, #86 on this list

It means praising or sticking up for yourself, and can have either positive or negative connotations, depending on the context.

Besides blowing your own horn/trumpet, you’ll also occasionally tooting or honking substituted for blowing there. Important men used to have heralds to announce their greatness, which is where the expression originally derived from. The metaphor is sometimes adapted to more modern cirumstances; for example, “honking a horn” refers to the horn on a motor vehicle, but the underlying sense of self-praise is unchanged.

And that’s not all...

There is one more modern use that’s even more exotic. Every now and again you encounter “blow one’s own horn” used in an extended metaphor, which put as delicately as possible, refers to the act of autofellatio. (And no, that auto- has nothing to do with motor vehicles. :) Eddie Murphy famously said he’d never leave home if he could blow his own horn, and this is what he meant by the expression.